1. Field of the Invention
The present invention relates to a resilient roll having a resilient cylindrical body formed of a rubber material, for example, and more particularly to a crowned resilient roll whose diameter continuously increases from its axially opposite ends toward its axially middle point. The present invention is also concerned with a method of producing such a crowned resilient roll.
2. Discussion of the Prior Art
In electrophotographic copying machines, printers or the like, there have been used various kinds of rolls which include: a charging roll for electrostatically charging a surface of a photoconductive drum; an image developing roll for developing an electrostatic latent image formed on the drum surface into a visible toner image; an image transfer roll for transferring the toner image onto a copy sheet; and an image fixing roll for fixing the toner image on the copy sheet. Each of these rolls has a resilient cylindrical body, through which a metallic center shaft extends so as to serve as a rotation axis of the roll. The roll is usually biased at axially opposite ends of the metallic shaft, against a mating roll such as a photoconductive drum, under a biasing force of springs or the like, so that the two rolls are rotated together, with the outer circumferential surfaces of the rolls being in contact with each other.
However, the resilient roll of the above type may be bent due to the biasing forces applied to its axially opposite ends when the roll is installed in position, or the roll per se may be slightly curved or have recesses in its surface. In such cases, a clearance is likely to appear between an axial middle portion of the roll and the mating photoconductive drum, for instance, resulting in poor or reduced rolling contact therebetween. If such a clearance exists between an electrostatically charging roll and a photoconductive drum, for example, an image produced may have defects due to poor charging, when the roll and drum are operated in the severe environment of low temperature and low humidity. To avoid this, the clearance or gap between the roll and drum needs to be controlled to be about 20 .mu.m or smaller
Even with a small clearance (of 20 .mu.m or smaller) between the charging roll and the photoconductive drum, poor charging may still occur due to an electrically insulating toner remaining on the drum. Namely, the remaining toner gradually accumulates on the drum surface during use, and forms insulating layers on local portions of the charging roll, which result in the poor charging. This may be avoided by provision of a cleaning member for the charging roll. In this case, however, the surface of the roll undesirably wears off due to sliding contact between the cleaning member and the charging roll. Accordingly, it is desirable to eliminate or zero the clearance between the charging roll and the photoconductive drum.
To achieve a good contact between the resilient roll and photoconductive drum, therefore, there has been proposed to use a crowned roll as shown in FIG. 7, which has a roll crown of several tens of microns. That is, the resilient roll is shaped such that the diameter of the axially middle portion of the roll is slightly larger (by several tens of microns) than those of its axially opposite end portions. In other words, the resilient roll is tapered from its axially middle point toward its axially opposite ends.
The resilient roll of this type is conventionally produced in the following manner. Initially, a suitable resilient material is vulcanized in a metal mold having a cylindrical cavity, with a metallic shaft 2 disposed at the center of the mold cavity, whereby a columnar resilient roll body 4 is formed on the shaft 2. Then, the surface of the roll body 4 is ground by a grinding machine, so that the roll body 4 has a crowned shape. For producing a charging roll, an electrically conductive resilient material is molded into the roll body 4, which is then grounded into a crowned shape. Then, the crowned roll body 4 is provided at its outer circumferential surface with a coating layer 6 as a resistance adjusting layer, which is formed of a semi-conductive resilient material. Further, a protective layer may be formed as needed on the surface of the coating layer 6. To form the coating layer 6, the surface of the roll body 4 is first cleaned, and is then evenly coated with the semi-conductive resilient material by a known dipping or roll-coating technique, such that the coating layer 6 has a constant thickness of several tens to hundreds of microns over the entire axial length of the roll.
To assure a good contact of the resilient roll of the above type with a photoconductive drum, for example, the roll body 4 is formed of a resilient material having a considerably low hardness (Hs: about 20.degree.-25.degree.). In this case, the ground roll surface tends to be rough, with minute pits and protrusions formed thereon. Namely, it is extremely difficult to grind the resilient roll of this type to provide a sufficiently smooth surface. Even after coating the roll body 4, the roll surface still has such minute pits and protrusions. Thus, the conventional resilient rolls produced in the above manner have a poor surface condition in which the minute pits are formed in the local areas of the roll surface. For practical use, these resilient rolls must be classified into different grades, depending upon the depth of the pits or the degree of the surface roughness.
During the grinding operation for the conventional crowned resilient roll, chips or particulates produced by the grinding are likely to stick to or accumulate on the roll surface since the low-hardness resilient body contains a comparatively large amount of softener. If these chips are not completely removed, the remaining chips form protrusions or other abnormality on local portions of the roll surface, which may possibly affect the performance of the resilient roll. It is therefore necessary to clean the roll surface after the grinding operation. Further, the roll body 4, which is formed of a low-hardness resilient material, must be ground at a low rate, resulting in an increased grinding time and reduced production efficiency.
Alternatively, the crowned roll may be produced in one step by using a metal mold with a crowned cavity having the same shape as the final product, and forming the roll body 4 in the mold by vulcanization. In this case, however, the metal mold must be split into two parts, inevitably causing burrs or flash generated at a joint of the two parts. This eventually necessitates a grinding process for the roll body obtained, leading to the same problems as described above.